Queue-differential backpressure scheduling and routing has been shown to be throughput optimal in terms of being able to stabilize the network under any feasible traffic rate vector. Additional research has extended the original result to show that backpressure techniques can be combined with utility optimization, resulting in simple, throughput-optimal, cross-layer network protocols for all kinds of networks. Recently, some of these techniques have been translated to practically implemented routing and rate-control protocols for wireless networks. The basic idea of backpressure mechanisms is to prioritize transmissions over links that have the highest queue differentials. Backpressure effectively makes packets flow through the network as though pulled by gravity towards the destination, which has the smallest queue size of zero.
Some communication networks such as intermittently connected mobile networks (ICMNs), e.g., encounter-based mobile networks or delay/disruption tolerant networks (DTNs), can use routing techniques that take in to consideration the intermittent availability of communication links when routing packets. ICMNs can be wireless networks with sparse mobile nodes in which complete connected end-to-end paths from source to destination rarely exists. As a result of this, traditional routing methods which rely on establishing paths would fail to deliver any packets. Several studies were conducted to overcome this problem by introducing mobility-assisted routing methods, such as single-copy and multi-copy techniques, that use mobility of the nodes to route packets. In these methods, the source stores the packet while moving until it encounters another node that has a better chance to deliver the packet to the destination. When that happens, the source hands the packet to that node which will be in charge to deliver the packet. This node also can encounter another one and so on. The moment of time that a node carries a particular packet is called the custodian of that packet.
Epidemic routing, or sometimes called flooding, in which nodes duplicate and transmit so both transmitter and receiver have copies of the same packet is guaranteed to support optimal delay in the case of having unrealistic infinite bandwidth. When bandwidth is limited however, epidemic routing suffers from large delay due to congestion resulting from large numbers of duplicates. Another method called Spray and Wait (S&W) creates a predefined fixed number of copies of the packet when admitted to the network. Those copies are distributed to distinct nodes and then each copy waits until it encounters the destination.